A new approach to estimate site amplification effects using observed ground motion records is proposed. First, we calculate the "bed rock spectra", adopting the omega-squared source model, frequency-dependent Q-factor, and free-surface effect. The source spectrum is given from seismic moment and corner frequency for each event, using observed records at rock site. The Q-factor is obtained from spectral inversion analysis. The site amplification, then, is defined as the ratio of observed seismic spectra to the "bed rock spectra" at each site. We apply this method to evaluate site amplification effects in Kansai area, Japan. The result are in good agreement with those obtained from the spectral inversion analysis so far done, as long as relative site amplification effects from both methods are compared. The spectral inversion method gives only site amplifications relative to a reference rock site. This method here proposed, instead, gives overall amplifications due to sedimentary layers overlying bed rock, even the site amplification effects caused by weathered layers on a rock site.
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